1. Field of the Invention
The invention pertains to the field of optical sensing devices.
2. Description of the Prior Art
It has been known in the prior art to use light sensors to sense the presence of preprinted registration marks on a moving web. There have been difficulties with prior art sensors such as those disclosed in U.S. Pat. Nos. 2,840,721 and 3,033,109.
The prior art sensors have had a serious drawback in that they needed to be located about 1/8" away from the moving web. With such a short distance from the web to the sensor, there have been instances where a web splice passing under the sensor would cause the web to brush against the sensor. Ink then is rubbed off the web onto the sensor with the result that the sensor will not function reliably. When the press is being set up or the web broken and the press is being re-webbed, the closeness of the prior art sensors to the web path has also been a problem. Thus, there has been a need for a sensor that can be positioned further away from the web but still reliably detect the marks on the web, than had been possible with the prior art sensors.